1. Field of the Invention
This invention relates to the use of phenolic resole resins having a free phenol content of about 15% to 25%, a molar ratio of not more than about 1.5 moles of aldehyde bound with one mole of a phenol in the resin, and lightburned magnesium oxide having a surface area of at least 20 square meters per gram, optionally with an ester functional hardening agent, which can be used with aggregates to produce raw batch compositions for use as refractories. The raw batch compositions exhibit good flow and compaction upon vibration, mix working times of at least 15 minutes and which harden as shaped articles in 24 hours or less at about 23.degree. C. with tensile strengths of at least 75 psi as measured at 25 C. The shaped articles can be thermally cured and further carbonized for use as refractories such as bricks or cast articles.
2. Prior Art
Currently cast-in-place monolithic refractory compositions are prepared by the use of hydraulic setting calcium aluminate cements. Alumina and magnesia (periclase) are major aggregate constituents but silicon carbide, silica, and graphite may also be present. The mixture of refractory cement, water, and aggregate is such that a fluid, pourable, easily vibratable mix results which is transferred to a mold, vibrated and allowed to harden at ambient temperature. Mix consistency and rate of hardening can be controlled by the type of refractory cement, amount of water used, use of dispersants and cement accelerators or retarders. Hardened shapes with mechanical strength result at room temperature, after thermal cure and after carbonizing at temperature in excess of 1,000.degree. C. (1,850.degree. F.). Increasing cement content, i.e., from 4% to 8%, increases strength. However, increasing cement (hydrated) content adversely affects refractory performance in the presence of molten metal such as steel and aluminum. Molten metal dissolves, softens or weakens one or more hydrated cement phases which in turn increases permeability of the hardened refractory shape. This in turn severely limits the service life of said shape.
The use of magnesium oxide for the room temperature hardening of phenolic resole resins is well known and disclosed in references such as: U.S. Pat. No. 2,869,191 of Jan. 20, 1959; U.S. Pat. No. 2,869,194 of Jan. 20, 1959; U.S. Pat. No. 2,869,196 of Jan. 20, 1959; and U.S. Pat. No. 2,913,787 of Nov. 24, 1959 which were issued to R. H. Cooper as well as U.S. Pat. No. 2,424,787 which issued to W. H. Adams on July 29, 1947.
Japanese Kokkai Tokyo Koho JP 60/90251 of May 21, 1985 to Kyushu Refractories Co. Ltd. discloses the room temperature hardening of resole with magnesium oxide and ethylene carbonate.
U.S. Pat. No. 4,794,051 of Dec. 27, 1988 to M. K. Gupta broadly discloses molding compositions comprising : (a) a phenolic resole resin; (b) a hardening agent of alkaline metal oxides or hydroxides e.g. magnesium oxide, and silanes; (c) a filler; (d) a lactone; and (e) a fiber reinforcement. However, the Gupta compositions do not harden at room temperature within 24 hours and possess still further shortcomings.
Phenolic resole resins such as those used in this invention have been used commercially for the manufacture of refractory bricks; however, such use did not involve hardening with magnesium hardening agents or magnesium hardening agents together with an ester hardening agent nor room temperature hardening as in the present invention.
U.S. Pat. No. 4,939,188 of Jul. 3, 1990 to A. H. Gerber discloses the use of lithium ion as an alkalizing agent for the room temperature cure of resoles for use as refractories and in a list of optional modifiers, magnesium oxide is mentioned.
European Patent Application, Publication No. 0094165 of 16-11-83 to Lemon et al. discloses the use of phenolic resole resins which are hardened with ester functional hardening agents in the production of paper filters. In the recitation of alkaline agents for condensation of the phenol and formaldehyde in the preparation of the resin, magnesium oxide is mentioned. This reference goes on to state that mixtures of alkalis can be used especially where one is used as a condensation catalyst and another to provide the remaining alkalinity to achieve the desired alkali to phenol molar ratio. However, sodium, potassium and lithium hydroxides are preferred because they give resin solutions in solvents such as water and lower alcohols which are more stable. U.S. Re 32,720 of Jul. 26, 1988 and U.S. Re 32,812 of Dec. 27, 1988 to Lemon et al. disclose room temperature hardening of alkaline phenolic resole resins with ester functional hardening agents for preparation of foundry molds and cores.
U.S. Pat. No. 4,831,067 of May 16, 1989 to Lemon et al. discloses hardening of a friction filler and alkaline phenolic resole resin with an ester functional hardening agent. Alkalis recited as suitable are sodium hydroxide or potassium hydroxide or a mixture of these with alkaline earth oxides such as magnesium oxide, etc.
European Patent Application, Publication No. 0202004 which was published on Nov. 11, 1986 and assigned to Foseco International Limited discloses production of refractory shapes by the use of esters to harden phenolic resole resins.
The above references and other prior art, suffer from one or more shortcomings in relation to the methods and compositions of the present invention. Among such shortcomings there can be mentioned: lack of adequate fluid mix consistency; lack of mix working life; lack of room temperature hardening capability within 24 hours; lack of sufficient mechanical strength after the room temperature hardening, thermal cure or carbonization; the use of high alkali metal content in the resin which acts as a fluxing agent when the refractory shape is fired at high temperature; the lack of high carbon producing binder; and the use of calcium based alkalis or combinations thereof with magnesium oxide which react quickly and reduce working time at room temperature.